In a recent letter (1) and Perspective (2) Zoback and Gorelick argued that carbon capture and storage (CCS) is a ‘likely unsuccessful strategy for significantly reducing greenhouse gas emissions’ because there is a high probability that earthquakes will be triggered by injection of large volumes of CO2, threatening the seal integrity of CO2 cap rocks.

Their conclusion assumes that the injection of CO2 will be unregulated. In fact, dedicated regulations have been created specifically to ensure that CCS is undertaken in an environmentally sound way, including to ensure that injection of CO2 does not compromise the integrity of the cap-rock. The risk of triggering seismicity by fluid injection or withdrawal is well-documented and must be property assessed and managed in any injection project. This is taken into account in the existing regulations for CO2 geological storage. Notable regulations with wide applicability being the new US EPA’s Class VI Rule for Carbon Dioxide Geologic Sequestration Wells (2010) (3); the European Union’s Directive for Geological Storage of Carbon Dioxide (2009/31/EC) (4); and the UNFCCC’s Modalities and Procedures for Carbon Dioxide Capture and Storage (Decision 10/CMP.7, 2011) (5). These restrict the allowable injection pressures so as to avoid the geomechanical scenarios discussed by Zoback and Gorelick.

Large-scale deployment of CCS will require the regulator to consider interactions among projects. This requirement exists in the US EPA rules, via the consideration of Area of Review, and in the European Directive for potential pressure interactions of sites in the same hydraulic unit. Regulators need to cumulate the impacts on pressure of many injection wells in many projects and model the aggregate pressure signal into the future. In cases where a seismically hazardous fault zone intersects the long-term large-scale area of slightly elevated pressure, the regulator should require responsive characterization, monitoring, and mitigation plans. Examples of characterization might be detailed assessment of the stress conditions near the fault zone; monitoring might be far-field pressure tracking during injection, and an example of mitigation might be brine withdrawal wells to diffuse the future pressure elevation if needed.

More research will be beneficial to further develop and provide tools which provide assurance as deployment of geologic sequestration grows to a widespread GHG reduction method. As noted by Zoback and Gorelick (1) some early projects favor sites with weak, ductile, and highly transmissive settings at which geomechanical risks are avoided, for example young sediments in the North Sea and the Gulf of Mexico. However, it is important also that incremental stages of injection into brittle, fractured, and faulted rocks are undertaken with appropriate monitoring of fault and fracture system response to pressure change so as to build experience in these settings and to calibrate predictive skills, and so assist regulatory decisions.

Hence the regulations for CCS are designed to ensure avoidance of the scenarios discussed by Zoback and Gorelick, and their paper and letter may be considered as a reminder to why CCS regulations contain such controls.

Tim Dixon,a,1 and Susan Hovorkab

a IEAGHG, Cheltenham, GL52 7RZ, UK

b Bureau of Economic Geology, The University of Texas at Austin, University Station, Box X, Austin, Texas 78713-8924